full-disclosure - Re: OpenSSH Security Advisory: gcmrekey.adv

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Date: Fri, 08 Nov 2013 19:56:36 +0100
From: CERT OPS Marienfeldt <cert.marienfeldt@...il.com>
To: coderman <coderman@...il.com>, 
 Full Disclosure <full-disclosure@...ts.grok.org.uk>
Subject: Re: OpenSSH Security Advisory: gcmrekey.adv

"If exploited, this vulnerability might permit code execution
        with the privileges of the authenticated user"

might explains the absence ;-)

Have a good one :-)




On 08.11.13 19:47, coderman wrote:
> surprised not a peep about this one here yet,... hmmm
>   a fun one ;)
> 
> we are accustomed to old software adding risk;
>  new (secondary effects of combined AUTH+ENC modes)
>    also carries risk!
> 
> ---
> 
> OpenSSH Security Advisory: gcmrekey.adv
> 
> This document may be found at: http://www.openssh.com/txt/gcmrekey.adv
> 
> 1. Vulnerability
> 
>         A memory corruption vulnerability exists in the post-
>         authentication sshd process when an AES-GCM cipher
>         (aes128-gcm@...nssh.com or aes256-gcm@...nssh.com) is
>         selected during kex exchange.
> 
>         If exploited, this vulnerability might permit code execution
>         with the privileges of the authenticated user and may
>         therefore allow bypassing restricted shell/command
>         configurations.
> 
> 2. Affected configurations
> 
>         OpenSSH 6.2 and OpenSSH 6.3 when built against an OpenSSL
>         that supports AES-GCM.
> 
> 3. Mitigation
> 
>         Disable AES-GCM in the server configuration. The following
>         sshd_config option will disable AES-GCM while leaving other
>         ciphers active:
> 
>         Ciphers
> aes128-ctr,aes192-ctr,aes256-ctr,aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc,aes256-cbc
> 
> 4. Details
> 
>         When using AES-GCM, sshd was not initialising a Message
>         Authentication Code (MAC) context that is unused when the
>         cipher mode offers authentication itself. This context
>         contains some callback pointers, including a cleanup callback
>         that was still being invoked during a rekeying operation.
>         As such, the address being called was derived from previous
>         heap contents.
> 
> This vulnerability is mitigated by the difficulty of
>         pre-loading the heap with a useful callback address and by
>         any platform address-space layout randomisation applied to
>         sshd and the shared libraries it depends upon.
> 
> 5. Credit
> 
>         This issue was identified by Markus Friedl (an OpenSSH
> developer) on November 7th, 2013.
> 
> 6. Fix
> 
>         OpenSSH 6.4 contains a fix for this vulnerability. Users who
>         prefer to continue to use OpenSSH 6.2 or 6.3 may apply this
>         patch:
> 
> Index: monitor_wrap.c
> ===================================================================
> RCS file: /cvs/src/usr.bin/ssh/monitor_wrap.c,v
> retrieving revision 1.76
> diff -u -p -u -r1.76 monitor_wrap.c
> --- monitor_wrap.c 17 May 2013 00:13:13 -0000 1.76
> +++ monitor_wrap.c 6 Nov 2013 16:31:26 -0000
> @@ -469,7 +469,7 @@ mm_newkeys_from_blob(u_char *blob, int b
>   buffer_init(&b);
>   buffer_append(&b, blob, blen);
> 
> - newkey = xmalloc(sizeof(*newkey));
> + newkey = xcalloc(1, sizeof(*newkey));
>   enc = &newkey->enc;
>   mac = &newkey->mac;
>   comp = &newkey->comp;
> 
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